Telomere shortening occurs during cell division, and results in cell senescence and death when a critical minimum length is reached. Telomere length is usually maintained by telomerase, an enzyme that is present in about 85 percent of human tumors, but seldom in normal somatic cells. This property makes telomerase an attractive therapeutic target;yet no successful telomerase-directed therapy has thus far been developed. A major reason is that pre-existing telomeres in tumor cells usually are of sufficient length to support multiple rounds of cell proliferation, so that telomerase inhibition is not cytotoxic before a lethal tumor burden is reached. The overall goal of the previous grant was to develop a strategy to use telomeres and telomerase as cancer therapeutic target. This goal has been achieved. Briefly, we established simultaneous targeting of telomeres and telomerase as s a useful therapeutic strategy. We found that paclitaxel causes direct damage to telomeres and induces telomerase activity, and that telomerase inhibitors, including hTR antisense and 3'-azido-3'deoxythymidine (AZT), enhanced paclitaxel activity in tumor cells and tumor-bearing animals without enhancing host toxicity. We further found that this strategy only works for chemotherapeutic drugs that damage telomeres and/or induce telomerase and only in tumors that depend on telomerase for telomere repair. Furthermore, the synergy between AZT and paclitaxel was diminished at higher AZT concentrations that are known to cause cell cycle arrest. The goal of the present application is to translate these earlier findings on the novel mechanisms of paclitaxel resistance and synergy between paclitaxel and AZT to useful clinical treatments. Aim 1 examines the generalizability of this strategy and provides additional molecular evidence of the role of telomere/telomerase in paclitaxel activity. Aim 2 will use PD- and computational modeling approaches to translate the preclinical data to find the AZT regimens that deliver to tumors the desired AZTTP levels and the optimal balance between the desired telomerase/telomere effects and the undesired cell cycle blockade effects, in humans. Aim 3, which is to test the strategy of extending the telomerase inhibitor treatment beyond the cytoreductive chemotherapy, may provide a means to control residual tumor cells and to sensitize tumor cells for subsequent chemotherapy. Aim 4 will identify the tumor types with the correct molecular target and response profile (i.e., tumors that depend on telomerase to repair paclitaxel-induced telomere damage and show synergy with AZT). This is necessary even though -85% of human tumors express telomerase because, as we have shown, not all telomerase expressing tumors are dependent on telomerase for telomere maintenance. Collectively, the proposed studies represent a rational therapy development paradigm, designed based on our newly discovered mechanism of paclitaxel resistance and synergy with telomerase inhibitors. Confirmation that the telomerase-mediated resistance is important for paclitaxel will support clinical development of the telomerase/telomere-targeting approach and may yield a novel treatment paradigm that maximizes the therapeutic value of paclitaxel.